Power amplifier saturation detection

1. A power amplifier circuit for a radio frequency (RF) transmitter, the power amplifier circuit comprising: a power amplifier having at least one output transistor having an output transistor terminal coupled to a supply voltage, the power amplifier including a high-band output transistor and a low-band output transistor selectably activatable in response to a band-select signal, the high-band output transistor having a high-band output transistor terminal coupled to a supply voltage, the low-band output transistor having a low-band output transistor terminal coupled to a supply voltage; a duty cycle detector having an input coupled to the output transistor terminal, the duty cycle detector including: a limiter section coupled to the output transistor terminal, the limiter section blocking positive voltage excursions and passing negative voltage excursions, the limiter section including a first limiter and a second limiter, the first limiter being coupled to the high-band output transistor terminal, the first limiter blocking positive voltage excursions and passing negative voltage excursions, the second limiter being coupled to the low-band output transistor terminal, the second limiter blocking positive voltage excursions and passing negative voltage excursions; and an averaging filter section coupled to an output of the limiter section, the averaging filter section including a first averaging filter coupled to an output of the first limiter and a second averaging filter coupled to an output of the second limiter; and a comparator section having first and second comparator inputs and producing a saturation detection output signal, one of the first and second comparator inputs being coupled to an output of the duty cycle detector, and the other of the first and second comparator inputs being coupled to a reference voltage circuit, the comparator section including a switching section responsive to the band-select signal, the switching section coupling the first comparator input to an output of the first averaging filter and coupling the second comparator input to a reference voltage circuit when the band-select signal indicates high-band operation, the switching section further coupling the first comparator input to an output of the second averaging filter and coupling the second comparator input to a reference voltage circuit when the band-select signal indicates low-band operation.

2. The power amplifier circuit claimed in claim 1 wherein: the first limiter includes a first diode and first biasing circuitry, the first biasing circuit biasing the first diode to turn on during a positive voltage excursion at the high-band output transistor terminal and turn off during a negative voltage excursion at the high-band output transistor terminal; and the second limiter includes a second diode and second biasing circuitry, the second biasing circuit biasing the second diode to turn on during a positive voltage excursion at the low-band output transistor terminal and turn off during a negative voltage excursion at the low-band output transistor terminal.

3. The power amplifier circuit claimed in claim 1 wherein: the first averaging filter includes at least one resistor and at least one capacitor; and the second averaging filter includes at least one resistor and at least one capacitor.

4. The power amplifier circuit claimed in claim 1 wherein the switching section comprises: a first switch responsive to the band-select signal, the first switch having a pole terminal coupled to the first comparator input, a first throw terminal coupled to the output of the first averaging filter and a portion of the reference voltage circuit, and a second throw terminal coupled to the output of the second averaging filter and a portion of the reference voltage circuit; a second switch responsive to the band-select signal, the second switch having a pole terminal output coupled to the second comparator input, a first throw terminal coupled to the output of the first averaging filter and a portion of the reference voltage circuit, and a second throw terminal coupled to the output of the second averaging filter and a portion of the reference voltage circuit.

5. The power amplifier circuit claimed in claim 4 wherein the reference voltage circuit comprises: a first current source responsive to the band-select signal and coupled to the output of the first averaging filter, the first throw terminal of the first switch device, and the first throw terminal of the second switch device; and a second current source responsive to the band-select signal and coupled to the output of the second averaging filter, the second throw terminal of the first switch device, and the second throw terminal of the second switch device.

6. A method for detecting power amplifier saturation in a radio frequency (RF) transmitter, the method comprising: amplifying an RF signal using an output transistor, the output transistor having an output transistor terminal coupled to a supply voltage, said amplifying including amplifying a high-hand RF signal using a high-band output transistor of a power amplifier and amplifying a low-band RF signal using a low-band output transistor, the high-band output transistor having a high-band output transistor terminal coupled to a supply voltage, the low-band output transistor having a low-band output transistor terminal coupled to a supply voltage; detecting a duty cycle of the amplified RF signal, said detecting including: limiting positive voltage excursions of the amplified RF signal while passing negative voltage excursions of the amplified RF signal to produce a limited signal, said limiting including limiting positive voltage excursions of the amplified high-band RF signal while passing negative voltage excursions of the amplified high-band RF signal to produce a first limited signal and limiting positive voltage excursions of the amplified low-band RF signal while passing negative voltage excursions of the amplified low-band RF signal to produce a second limited signal; and averaging the limited signal to produce an averaged signal, said averaging including averaging the first limited signal to produce a first averaged signal and averaging the second limited signal to produce a second averaged signal; providing a signal representing the duty cycle to a first input of a comparator section and a reference voltage to a second input of the comparator section, said providing including coupling the first averaged signal to the first input of a comparator section and coupling a reference voltage to the second input of the comparator section when a band-select signal indicates high-band operation, and coupling the second averaged signal to the second input of the comparator section and coupling a reference voltage to the first input of the comparator section when the band-select signal indicates low-band operation; and producing a saturation detection output signal in response to a comparison of the signal representing the duty cycle with the reference voltage.

7. The method claimed in claim 6 wherein: limiting positive voltage excursions of the amplified high-band RF signal while passing negative voltage excursions of the amplified high-band RF signal to produce a first limited signal includes a first diode turning on during a positive voltage excursion and turning off during a negative voltage excursion; and limiting positive voltage excursions of the amplified low-band RF signal while passing negative voltage excursions of the amplified low-band RF signal to produce a second limited signal includes a second diode turning on during a positive voltage excursion and turning off during a negative voltage excursion.

8. The method claimed in claim 6 wherein: averaging the first limited signal includes low-pass filtering the first limited signal; and averaging the second limited signal includes low-pass filtering the second limited signal.

9. A mobile wireless telecommunication device, comprising: a user interface; an antenna; a baseband subsystem coupled to the user interface; and a radio frequency (RF) subsystem coupled to the baseband subsystem and the antenna, the RF subsystem including a transmitter portion and a receiver portion, the transmitter portion including a modulator, an up converter and a power amplifier system, the power amplifier system including: a power amplifier having a high-band output transistor and a low-band output transistor, the high-band output transistor having a high-band output transistor terminal coupled to a supply voltage, the low-band output transistor having a low-band output transistor terminal coupled to a supply voltage; a first limiter section coupled to the high-band output transistor terminal, the first limiter section blocking positive voltage excursions and limiting negative voltage excursions; a first averaging filter section coupled to an output of the first limiter section; a second limiter section coupled to the low-band output transistor terminal, the second limiter section blocking positive voltage excursions and passing negative voltage excursions; a second averaging filter section coupled to an output of the second limiter section; and a comparator section, the comparator section having first and second comparator inputs and producing a saturation detection output signal, the comparator circuit further including a switching section responsive to a band-select signal, the switching section coupling the first comparator input to an output of the first averaging filter and coupling the second comparator input to a reference voltage circuit when the band-select signal indicates high-band operation, the switching section further coupling the first comparator input to an output of the second averaging filter and coupling the second comparator input to a reference voltage circuit when the band-select signal indicates low-band operation.

10. The mobile wireless telecommunication device claimed in claim 9 wherein: the first limiter section includes a first diode and first biasing circuitry, the first biasing circuit biasing the first diode to turn on during a positive voltage excursion at the high-band output transistor terminal and turn off during a negative voltage excursion at the high-band output transistor terminal; and the second limiter section includes a second diode and second biasing circuitry, the second biasing circuit biasing the second diode to turn on during a positive voltage excursion at the low-band output transistor terminal and turn off during a negative voltage excursion at the low-band output transistor terminal.

11. The mobile wireless telecommunication device claimed in claim 9 wherein: the first averaging filter section includes at least one resistor and at least one capacitor; and the second averaging filter section includes at least one resistor and at least one capacitor.

12. The mobile wireless telecommunication device claimed in claim 9 wherein the switching section comprises: a first switch responsive to the band-select signal, the first switch having a pole terminal coupled to the first comparator input, a first throw terminal coupled to the output of the first averaging filter section and a portion of the reference voltage circuit, and a second throw terminal coupled to the output of the second averaging filter section and a portion of the reference voltage circuit; a second switch responsive to the band-select signal, the second switch having a pole terminal output coupled to the second comparator input, a first throw terminal coupled to the output of the first averaging filter section and a portion of the reference voltage circuit, and a second throw terminal coupled to the output of the second averaging filter section and a portion of the reference voltage circuit.

13. The mobile wireless telecommunication device claimed in claim 12 wherein the reference voltage circuit comprises: a first current source responsive to the band-select signal and coupled to the output of the first averaging filter, the first throw terminal of the first switch device, and the first throw terminal of the second switch; and a second current source responsive to the band-select signal and coupled to the output of the second averaging filter, the second throw terminal of the first switch device, and the second throw terminal of the second switch.